Remove low interest ClanLib modules

This patch removes the following ClanLib modules:
GUI (replaced by uicore)
CSSLayout (no longer needed when GUI is gone)
GameIDE (no community interest)
Physics2D (no maintainer)
Physics3D (no community interest)
Scene3D (no community interest)
SWRender (no community interest; obsolete - we live in a GPU world now)
Database (no community interest)
Sqlite (no community interest)
Compute (no community interest; semi-obsolete in gaming context now that OpenGL has compute shaders)